Spectroscopic electron microscope wherein a specimen is irradiated with x-rays and the electrons emitted are energy analyzed



United States Patent ()fiice 3,374,346 SPECTRGSCOPIC ELECTRON MICROSCOPEWHEREIN A SPECIMEN IS IRRADIATED WITH X-RAYS AND THE ELECTRGNS EMI'ITEDARE ENERGY ANALYZED Hiroshi Watanabe, Kokubunji-shi, Japan, assiguor toHitachi, Ltd., Tokyo, Japan 7 Filed July 12, 1965, Sal. No. 470,991Claims priority, application Japan, July 15, 1964, 39/40,591 3 Claims.(Cl. 250-495) This invention is concerned with spectroscopic electronmicroscopes.

Hereto fore, X-ray fluoresence spectrometers have often been used inanalizing specimens of unknown composition and in which the specimen isirradiated with X-rays and the secondary X-rays thus generated areanalized for wavelengths. The use of such devices, however, is limitedto elements of atomic numbers of the order of Mg or heavier sincelighter elements produce characteristic X-rays of such long wavelengthsas to make their spectroscopy practically infeasible. Thus, in the past,microanalysis of elements lighter than lMg has been impossible employingany nondestructive technique of X-ray analysis. Even with elementsheavier than Mg, X-ray microanalizers exhibit only a resolution of theorder of Lu. In addition, they involve 'a deficiency that the specimensurface is inevitably contaminated as it is directly irradiated by anelectron beam.

'Under these circumstances, the present invention is intended to providea novel spectroscopic electron microscope which is free from any of thedifficulties previously met and to attain this objective proposes toobtain an image of the X-ray-irradiated portion of the specimen by useof electron rays of a particular energy level selected from thoseemitted by the specimen when it is irradiated with X-rays of apredetermined Wavelength.

The foregoing and other objects, features and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings, in which:

FIG. 1 is an energy diagram illustrating the principles of theinvention;

FIG. 2 is a schematic diagram of an electron microscope embodying theinvention; and

FIG. 3 schematically illustrates another embodiment of the invention.

Referring first to FIG. 1, the principles of the present invention willbe described in detail. It has been found that any unknown specimen canbe determined by energy analysis of the K-, L- or M-electrons ejectedout of their orbits in the atoms which constitute the X-rayirradiatedportion of the specimen, and further by determining the difference ofthe energy of the ejected electrons from the energy h'y of the impingingX-rays, which corresponds to the energy of K-, L- or M-electrons as heldin the atoms. Now suppose that a specimen of aluminum is irradiated withX-rays having an energy of hy. In this case, the orbits of the K- andL-levels are filled with electrons while the M-level orbit has a vacancyincluding only three electrons. Thus, the electrons of the M-level formthe outer shell electrons. Let it now be assumed that K-electrons areejected by X-radiation. Then the electrons will run through the freespace with an energy corresponding to that indicated by E, in FIG. 1. Itwill thus be apparent that the specimen can readily be identified asaluminum by accurately measuring the value E, by means of an energyanalyzer and referring to the table of E in FIG. 1.

Further, by selecting from the electrons ejected by the specimen onlythose having the particular energy value 3,374,346 Patented Mar. 19,1968 for formation of a desired microscopic image, it will beappreciated that the distribution of the specimen components can beclearly observed as a contrast between light and shade in the image.

Reference will next be made to FIG. 2, which illustrates an electronmicroscope embodying the principles of the present invention.

In FIG. 2, reference numeral 1 indicates an electron beam emitted by theelectron gun (not shown) of the electron microscope; 2 indicates ananticathode target formed of Cu, W or other pure metal selectedaccording to the purpose; 2' indicates characteristic X-rays formed atthe target 2 and impinging against a specimen 3 under examination; 3'indicates an electronbeam of K-, ,L, M- or other level emitting from thespecimen 3; 4 indicates an objective lens; 5 indicates a first deflectormeans for scanning the electron beam 3; 6 indicates a first stop oraperture diaphragm in having a slit or tiny aperture; 7 indicates animage of the specimen formed by the objective lens 4; 8 indicates andelectron-energy analyzing lens; 9 indicates a second stop having a slitor tiny aperture; 19 and 11 indicate analyzed electron images; 12indicates a cylindrical electron lens; 13 indicates a second deflectorfor scanning; 14 indicates an image plane; and 15 indicates the finalimage of the specimen.

The electron beam 3' emitting from the specimen when it is irradiated byX-r'ays of a predetermined wavelength apparently includes scatteredelectrons having different energy values E E E etc. Assume that thefirst deflector 5 is subjected to a first deflecting voltage V such asshown in the left portion of FIG. 2 for the purpose of deflecting thespecimen image 7 formed in the plane of the first stop 6. Then, theimage 7 is displaced into a position indicated at 7', for example, atthe instant of t=0 and into position 7" at the instant of t=1-,-rrepresenting half of the period of the deflecting voltage. Accordingly,the electron ray passing through the slit of the first stop 6 at theinstant of t=0 includes electrons form ing one end of the image (i.e.,the tail end of the arrow indicating the image) while the electron raypassing through the slit at the instant of 1:1- includes electronsforming the other end of the image (i.e. the head end of the arrow).Thus, it is noted that the electrons corresponding to the respectiveimage points and passing through the slit of the first stop 6 form apattern of energy distribution including values E E E etc., which can beanalyzed by means of the analyzing lens 8. In this embodiment, theanalyzing lens 8 takes the form of an electrostatic unipotential lenshaving a rectangular aperture and use is made of the extra-ordinarilylarge chromatic aberration of such lens for energy analysis of theelectron ray. It is to be understood, however, that the analyzing lens 8may take any other form of electron prism which is effective to dispersethe electrons in accordance with their energy difierences. Apparently,the same purpose may be served by a pair of electron deflector plates.Incidentally, even though the analyzing lens 8 has a substantially highresolution, it is difficult to obtain a deflection of the order enoughto identify the specimen image as a whole and also the energy deviatitTnof the electron ray cannot be so large. This is the reason why the firstdeflector 5 is used to divide the specimen image 7 into minute sectionsand the energies of the image-forming electrons are analyzed separatelyfor each of the individual image sections. In this manner, it will beunderstood that any specimen image can be converted by energy analysisinto an image formed solely by elec trons of the same energy levelirrespective of the size of the specimen image. Further, by such energyanalysis of each of the electron rays, which correspond to therespective image points, it is possible to select from the electron rayshaving an energy distribution including E E etc. only those having anydesired energy value and the specimen can thus be identified by findingthe value E as described hereinbefore.

Again referring to FIG. 2, it is to be noted that, at the instant of t=in the diagram of the first deflecting voltage V elastically scatteredelectrons of energy E included in the electron ray forming the tailportion of the specimen image now positioned at 7', are exclusivelyallowed to preceed through the slit of the second stop 9, as indicatedat and inelastically scattered electrons of energy E included in suchelectron ray are thrown onto the second stop 9 around the slit thereinas indicated at 11'. Similarly, at the instant of t=1-, electrons ofenergy E included in the electron ray this time forming the head portionof the specimen image at 7" are projected as indicated at 10" but thoseof energy E; are projected as indicated at 11". Thus, only rays ofelastically scattered electrons, having one and the same energy value Bare allowed to pass through the slit of the second stop 9. In thismanner, even with a specimen image of a substantial size, it is possibleto select from the electron rays corresponding to the respective imagepoints only those scattered elastically and having an energy value E forpassage through the aperture in the second slit. The electron rays ofthe same energy E having passed through the second stop 9, are deflectedto form a desired final image 15 of the specimen under examination onthe image plane 14 by the second deflector 13, to which a seconddeflecting voltage V is applied which is opposite in polarity to thefirst deflecting voltage V as shown in the left portion of FIG. 2. Itwill readily be appreciated that the final image of the specimen isextraordinarily high in quality since it is formed solely by elasticallyscattered electrons having the same energy value E The cylindricalelectron lens 12 arranged beneath the second stop 9 serves to correctany substantial astigmatism of the analyzing lens 8 for the purpose offurther enhancing the quality of the image 15. When it is desired toobtain a higher magnification, an appropriate magnifying lens system notshown may be used in combination to further magnify the image 15.

With the embodiment described above, it will readily be understood thata specimen image formed of monoenergetic electrons having any desiredenergy value other than E, can be obtained by properly adjusting theenergyanalizing power of the analyzing lens 8 or the position of theslit of the second stop 9 or both to allow passage of electron rays ofthe energy value only.

Reference will next be made to FIG. 3, which illustrates anotherembodiment of the present invention. In this figure, reference numerals16, 16', 16", indicate an electron diffraction pattern obtained in theback focal plane of the objective lens 4; 4 indicates a first auxiliarylens; 4" indicates a first stop or aperture diaphragm adjustable inposition; 17, 17', 17", indicate enlarged diffraction spots formed inthe plane of the first stop 4"; 18 indicates an analyzing lens; 19indicates a second stop or aperture diaphragm also adjustable inposition; 20, 20', 20", indicate analyzed images formed in the plane ofthe second stop -19 by energy analysis of the analyzing lens 18; 21indicates a magnifying or second auxiliary lens; 22 indicates an imagingplane; and 23 indicates a final image.

As described in connection with the first embodiment of the presentinvention, the electron beam emitting from the specimen 3 includeselectrons having different energy values E E E etc. Accordingly, theelectron rays forming the respective image points 17, 17', 17" of theelectron diffraction pattern, which is formed in the plane of the firststop 4" as a magnified projection of the electron diffraction pattern16, 16', 16", obtained in the back focal plane of the objective lensthrough the auxiliary lens 15, have an energy distribution includingenergy values of E E E etc. Suppose that the aperture position of thefirst stop 4" is adjusted, for example, to allow passage therethrough ofthose electrons which form the central diffraction spot 17, asillustrated. Such electrons are energy-analyzed by means of theanalyzing lens 18 to form in the plane of the second stop images 20, 20,20", corresponding to the respective energy values E E E in the form ofa discontinuous spectrum. It will be noted that, of all the electronsthus analyzed, those having the energy value of E, are elasticallyscattered electrons and all the rest are inelastically scattered. Theanalyzing lens 18 in this embodiment is in the form of an electrostaticunipotential lens which and thus exhibits 'an extraordinarily largechromatic aberration, which is utilized for energy analysis. Theanalyzing lens '18, however, may apparently take any other form ofelectron prism which is effective to disperse the electrons inaccordance with the energy difference therebetween. For example, thesame purpose may be served by 'a pair of electron deflector plates, asdescribed 'hereinbefore in connection with the preceding embodiment ofthe invention.

In any case, the images formed in the plane of the second stop 19 indifferent positions corresponding to the respective energy values areeach formed by electrons which are monoenergetic having the same energyvalue. It is possible, therefore, to obtain on the imaging plane 22 afinal image 23 of high quality, which is formed solely by elasticallyscattered electrons of energy value E and magnified by the magnifyinglens 21, by adjusting the aperture position of the second stop 19 so asto allow passage therethrough of only the elastically scatteredelectrons of energy E for example, forming the diffraction spot 16, asillustrated. Now, take the electron rays 1' which deviate from the beam1 travelling parallel to the axis 0-0 of the microscope by only a slightangle, which with ordinary electron microscopes ranges from l() to lOradian. Since an image of any particular point of the specimen, which inthe figure is shown as the headpoint of the arrow, is apparently formedat the points of intersection of electron rays emitting from the pointof the specimen, such image is formed in the vicinity of theintermediate or first auxiliary lens 15 and then of the analyzing lensand finally is formed in the imaging plane 22 through the intermediaryof the projection or second auxiliary lens 21.

Now, any one of the diffraction spots, for example, the central spot 16includes all the electron rays emitting from the different points of thespecimen and travelling parallel to the axis O-O. Thus, of all theinformations carried on a specimen of limited extent, the informationcarried by the electron rays emitting in the direction of the axis 0-0of the microscope is all included in the central spot 16. Accordingly,with the electron microscope, a bright field image can be obtained byplacing the objective aperture diaphragm in the position of the centralspot 16 while a dark field image can be obtained by placing thediaphragm in the position of each of the other spots 16', 16",

In the above example, the energy analysis of the diffraction spots 16,16', 16", is useful in that it enables energy selection of the finalimage as obtained on the electron microscope. That is, it is possible toobtain a mono-energetic image on an electron microscope by energyanalysis of the diffraction spots each formed therein as a gathering ofelectron rays, as described hereinbefore. For example, the electron rayspassing through the aperture of the second stop 19, for example,positioned as illustrated are monoenergetic having the energy value of Eand include only electrons emitting from the different points of thespecimen in parallel with the axis of the microscopic system. Thus, itwill be apparent that by employing such electron rays, a microscopicimage of bright field can be obtained which is monoenergetic includingelectrons of the same energy of B Of course, electron rays slightlyinclined to the microscopic axis and having the energy value of B arealso allowed to pass the aperture of the second stop 19, which islimited in magnitude, thereby contributing to the focusing of a finalimage in the electron microscope.

In this manner, of all the electron rays emitting from the differentpoints of the specimen in parallel to the microscopic axis or at aslight inclination thereto, only those having the energy value of E, areallowed to pass through the aperture of the second stop 19 and areprojected on the final imaging plane 22 through the magnifying lens 21.On this occasion, the electron rays emitting from the different pointsof the specimen are all focused to form respective points of the finalimage 23, and thus the latter is apparently formed only by electron rayshaving the energy value E Next assume that the second stop 19 isdisplaced to the right as viewed in FIG. 3 or the operating point of theanalyzing lens is slightly changed so that the aperture position of thesecond stop 19 is slightly varied relatively to the position of theanalyzed image 20, 20', 20", thereby to allow only the spot formed byelectrons having an energy value other than E for example, the spot 20formed by electrons of energy E to be placed in the aperture of thesecond stop 19. It will be apparent that the image 23 thus obtained inthe imaging plane of the electron microscope is one formed only byinelastically scattered electrons having the energy value of E Accordingto the present invention, it will be appreciated from the foregoing thatmicroanalysis can be performed successfully not only with heavierelements but also with such lighter elements as cannot be analyzed byany X-ray microanalyzer or X-ray spectrograph for fluorescence analysisand that a resolution is obtainable which corresponds to that of anyordinary electron microscope. In addition, the distribution of thespecimen components can be directly observed as the light-and-darkcontrast of a final image formed in the electron microscope. A furtherpractical advantage of the present invention is that, since the specimenis only irradiated with X-rays, there is no danger of its surface beingcontaminated as with the case of the X-ray microanalyzer and thus thecomponent distribution of any minute specimen can be observed withhigher accuracy.

What is claimed is:

1. A spectroscopic electron microscope comprising means for irradiatingthe specimen under examination with X-rays of a predeterminedwavelength, an objective lens, and an axial arrangement between theobjective lens and the imaging plane of the microscope of componentsincluding:

a first deflecting device for deflecting the electron image of thespecimen formed by the objective lens, a first aperture diaphragm havinga small aperture or slit and disposed in the imaging plane of theobjective lens, means for energy analysis of the electrons passing intime sequence through said first aperture diaphragm to form therespective points of said image of the specimen, a second aperturediaphragm adapted to allow passage of only those electrons having anydesired energy value of all the electrons analyzed by said means forenergy analysis, and a second deflecting device for deflecting the raysof monoenergetic electrons passing through said second aperturediaphragm toward their respective positions in the imaging plane of themicroscope.

2. A spectroscopic electron microscope comprising means for irradiatingthe specimen under examination with X-rays of a predeterminedwavelength, an objective lens and an axial arrangement between theobjective lens and the imaging plane of the microscope of componentsincluding:

a first deflecting device for deflecting the electron image of thespecimen formed by said objective lens, a first aperture diaphragmhaving a small aperture or slit and disposed in the imaging plane ofsaid objective lens, means for energy analysis of the electrons passingthrough said first aperture diaphragm in time sequence according to thedeflection period of said first deflecting device to form the respectivepoints of said image of the specimen, a second diaphragm apertureadapted to allow passage of only those electrons having any desiredenergy value of all the electrons analyzed by said means for energyanalysis, means for correcting the electron rays passing through saidsecond aperture diaphragm for the astigmatism of said energy analysismeans, and a second deflecting device for deflecting the electron raysthus corrected toward their respective positions in the imaging plane ofthe microscope.

3. A spectroscopic electron microscope comprising means for irradiatingthe specimen under examination with X-rays of a predeterminedwavelength, an objective lens and an axial arrangement between saidobjective lens and the imaging plane of the microscope of componentsincluding:

a first auxiliary lens for forming a magnified projection of theelectron dilfraction spots of the specimen formed in the back focalplane of said objective lens, first selecting means for selectivelyallowing passage of the electrons forming the magnified diffractionspots, means for energy analysis of the electrons selected by said firstselecting means, second selecting means for allowing passage of thoseelectrons having any desired energy value of all the electrons analyzedby said energy analysis means, and a second auxiliary lens for forming amagnified projection of the electrons selected by said second selectingmeans.

No references cited. ARCHIE R. BORCHELT, Primary Examiner. RALPH G.NILSON, Examiner.

A. L. BIRCH, Assistant Examiner.

1. A SPECTROSCOPIC ELECTRON MICROSCOPE COMPRISING MEANS FOR IRRADIATINGTHE SPECIMEN UNDER EXAMINATION WITH X-RAYS OF A PREDETERMINEDWAVELENGTH, AN OBJECTIVE LENS, AND AN AXIAL ARRANGEMENT BETWEEN THEOBJECTIVE LENS AND THE IMAGING PLANE OF THE MICROSCOPE OF COMPONENTSINCLUDING: A FIRST DEFLECTING DEVICE FOR DEFLECTING THE ELECTRON IMAGEOF THE SPECIMEN FORMED BY THE OBJECTIVE LENS, A FIRST APERTURE DIAPHRAGMHAVING A SMALL APERTURE OR SLIT AND DISPOSED IN THE IMAGING PLANE OF THEOBJECTIVE LENS, MEANS FOR ENERGY ANALYSIS OF THE ELECTRONS PASSING INTIME SEQUENCE THROUGH SAID FIRST APERTURE DIAPHRAGM TO FORM THERESPECTIVE POINTS OF SAID IMAGE OF THE SPECIMEN, A SECOND APERTUREDIAPHRAGM ADAPTED TO ALLOW PASSAGE OF ONLY THOSE ELECTRONS HAVING ANYDESIRED ENERGY VALUE OF ALL THE ELECTRONS ANALYZED BY SAID MEANS FORENERGY ANALYSIS, AND A SECOND DEFLECTING DEVICE FOR DEFLECTING THE RAYSOF MONOENERGETIC ELECTRONS PASSING THROUGH SAID SECOND APERTUREDIAPHRAGM TOWARD THEIR RESPECTIVE POSITIONS IN THE IMAGING PLANE OF THEMICROSCOPE.